1. Field of the Invention
The present invention is related to the field of high-explosive shaped charges. More particularly, the present invention is related to a design for the exterior cover or lid of a shaped charge used for perforating earth boreholes having protective pipe, or casing.
2. Discussion of the Related Art
High-explosive shaped charges are commonly used to create hydraulic communication passages, called perforations, in a protective pipe, or casing, used in completing a borehole penetrating an earth formation. The charges are typically conveyed to a depth of interest in a borehole by means of wireline or pipe.
It is often desirable to create a plurality of perforations in the casing all at one time, so various devices have been devised for conveying a plurality of charges to the depth of interest. The devices are known as gun assemblies, or guns. Some guns are of a type known as fully-expendable, a term used to describe a type of gun which enables recovery of substantially no useful components of the gun after detonation of the charges.
One type of fully expendable gun assembly is known as an expendable barcarrier. Compared with some other types of guns sized to enable transport through a tubing string which may be coaxially inserted within the casing, expendable bar carriers have the advantages of: enabling transport of larger charges in the gun assembly; and enabling recovery of greater amounts of expended charge debris after detonation of the gun.
Expendable bar carrier gun assemblies typically comprise shaped charges which are sealed to exclude fluids which may be present in the borehole before detonation of the gun. The sealing means typically includes a housing containing the explosive, and a lid disposed on the side of the charge from which the explosive detonation discharge is expelled. The lid typically has a sealing surface which engages a mating sealing surface in the housing and is sealed by an o-ring or similar seal which enables relative rotation between the lid and the housing. The lid also has a threaded mounting stud for threadedly engaging a carrier.
The carrier is typically an elongated steel bar having a plurality of threaded holes located colinearly at spaced apart locations along the length of the carrier. The holes have threads sized to engage the studs on the charge lids. Typically the carrier will have 4 or 6 threaded holes per foot of carrier length. Carrier lengths can be as short as one foot, or in some cases extend to 100 feet or more when carriers are assembled from sections, typically each section being ten feet long. Some or all of the holes in each carrier can have charges mounted therein on any particular gun assembly, depending on the completion requirements of the particular borehole.
One of the problems encountered with bar carrier guns is that the charge lids typically have a round exterior surface adjacent to the stud, and therefore do not have a suitable surface to which stud mounting torque can be applied. Several specialized tools have been developed to apply mounting torque to the lid, but the available tools have neither a means to measure the applied torque nor a means to mount a plurality of charges simultaneously. Other methods of mounting charges in bar carriers include torquing with pipe wrenches or pliers. Both pliers and pipe wrenches can distort the sealing surface of the charge lid, leading to leakage of the lid when the gun is lowered into the borehole, and consequent possible failure of the charge to detonate correctly.
It is desirable to retain as many lids as possible in the carrier after detonation of the charges, so that the amount of debris left in the borehole as a result of detonating the gun is kept to a minimum. If insufficient mounting torque is applied to the charges, detonation can cause ejection of the charge lids from the carrier.
It is also desirable to mount a plurality of charges in the carrier simultaneously to reduce the gun assembly time. Round lids known in the art make it difficult to use an apparatus which would enable simultaneous torquing of a plurality of charges.
It is an object of the present invention to provide a shaped charge having a mounting surface which enables application of mounting torque without distorting the lid of the charge.
It is a further object of the present invention to provide an apparatus which can apply mounting torque simultaneously to a plurality of shaped charges on an expendable bar carrier borehole perforating gun assembly.
It is yet a further object of the present invention to provide a lid for a shaped charge used in an expendable bar carrier gun assembly which enables improved retention of the lids in the carrier after detonation of the gun assembly.